high rpm fluctuation

I have a 2003 Chevy Cavalier. While in park and pressing the accelerator gradually, the rpms fluctuate after reaching 4500 rpms. Once the fluctuating starts, it drops to 4000 then back to 4500 rpm over and over again and will not exceed 4500 rpm no matter how much the accelerator is depressed. Rpm does not fluctuate at any point less than 4000 rpm. A mechanic told me to change out the idle control but unfortunately the problem still exists. What suggestions does anyone have? thanks